This invention relates generally to vortex shedding flowmeters, and more particularly to a low-cost flowmeter of this type capable of measuring both liquid and gas flow rates with exceptional accuracy.
When a fluid such as gasoline or natural gas is to be metered, the need for precise measurement becomes more pressing as the market price of the fluid increases. Thus when gasoline or fuel oil was relatively cheap, one was more tolerant of small metering errors, for these were not reflected in significant billing differences. But with the recent sharp rise in the cost of gasoline and fuel oil, purchasers are now insistent that these liquids be more precisely metered so that they are only called upon to pay for the exact amount of liquid supplied to them and no more. By the same token, suppliers of costly liquids also demand precise readings, for flow reading errors may underestimate the amounts supplied to customers and therefore be to the disadvantage of suppliers.
Flowmeters in present use which are capable of affording accurate flow rate readings are mainly of the turbine or of the positive displacement type. In a turbine meter, a rotor lies in the path of a stream which impinges on the rotor blades, imparting a force thereto setting the rotor in motion at a speed proportional to fluid velocity. In a positive-displacement meter, a measuring disc is caused to move in a nutating motion by an advancing volume of liquid.
While both the turbine meter and the positive-displacement meter afford accurate flow measurements when in good working condition, because they make use of moving parts which are subject to fouling by contaminants carried by the fluid being metered; and because the bearings or supports for the moving parts are subject to wear, their accuracy may be impaired by these factors. Moreover, mechanically-operated meters are relatively expensive, the cost of these meters escalating dramatically with more stringent accuracy requirements.
The concern of the present invention is with low-cost vortex-shedding flowmeters capable of metering gas or liquids with a high degree of accuracy. While vortex-shedding flowmeters of the type heretofore known represent a no-moving-parts alternative to turbine and positive displacement meters and do not suffer from the mechanical drawbacks of such meters, the fact is that in terms of accuracy, they fall considerably short of turbine and positive displacement meter performance. Hence before considering the structure and function of a highly linear vortex-shedding flowmeter in accordance with the invention, we shall first briefly review the present state of the art of vortex-shedding meters.
It is well known that under certain circumstances the presence of an obstacle in a flow conduit will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed. These patterns are known as Karman vortex streets. The frequency at which vortices are shed in a Karman vortex street is a function of flow rate. It is this phenomenon which is exploited to create a vortex-shedding flowmeter to measure the volumetric flow of fluids being treated or supplied in order to carry out various control functions. Flowmeters of this type are disclosed in the Bird patent, U.S. Pat. No. 3,116,639, and in the White patent, U.S. Pat. No. 3,650,152. Existing flowmeters of the vortex shedding type are capable of effecting volumetric or mass flow measurement.
In a vortex-shedding flowmeter, the frequency of shedding is proportional to the velocity of fluid passing through the flow tube containing the shedding body, but only as long as the separation point from which shedding takes place remains fixed and the feedback mechanism causing shedding to transfer from one side of the body to the other remains constant.
In its most elementary form, the shedding body is a simple cylinder mounted across the flow tube. The difficulty experienced with this type of shedding body is that the separation point (i.e., the location at which vortices leave the body) shifts with Reynolds numbers. As a consequence, the vortex trail tends to meander down the flow tube behind the shedding body. If the angle of this vortex trail changes, the feedback mechanism causing shedding to take place from alternate sides of the shedding body also undergoes change, thereby giving rise to deviations from the predicted frequency of the shedder. As a result, meter accuracy and meter repeatability are poor.
Vortex meters are commercially available having shedding bodies which are designed to overcome these drawbacks by optimizing the shedding body width and geometry in relation to the flow tube size. The U.S. Pat. No. 3,572,117 to Rodely discloses a bluff body flowmeter having a prescribed geometric configuration designed to minimize irregularities in the oscillating wake. These meters constitute and improvement over meters having cylindrical shedding bodies. However, under less-than-ideal operating conditions, the vortex wake or trail created by these non-cylindrical shedding bodies will still, on occasion, become intermittent or meander, to produce the same disadvantages encountered with cylindrical bodies.
The Burgess U.S. Pat. No. 3,589,185 discloses an improved form of vortex-type flowmeter wherein the signal derived from the fluid oscillation is relatively strong and stable to afford a favorable signal-to-noise ratio insuring accurate flow-rate information over a fairly broad range. In this meter, an obstacle assembly is mounted in the flow conduit, the assembly being constituted by a block positioned across the conduit with its longitudinal axis at right angles to the direction of fluid flow, a strip being mounted across the conduit behind the block and being spaced therefrom to define a gap which serves to trap Karman vortices and to strengthen and stabilize the vortex street. This street is sensed to produce a signal whose frequency is proportional to flow rate.
In another Burgess Patent, U.S. Pat. No. 3,888,120, dealing with a vortex-type flowmeter, there is disclosed an obstacle assembly constituted by a fixed front section contoured to cause flow separation of the incoming fluid stream whose flow rate is to be measured, and a rear non-streamlined section which is shaped to interfere with the vortex street in the wake of the front section and is cantilevered from the front section to define a gap. The rear section is slightly deflectable relative to the front section whereby it is excited into minute vibrations by the vortex street. These vibrations are sensed by a strain gauge to produce a signal proportional to flow rate.
The liquid vortex flowmeter Model 10 LV 1000, manufactured by the Fischer & Porter Company of Warminster, Pa., the assignee herein, operates in accordance with the principles set forth in Burgess U.S. Pat. No. 3,888,120. This liquid vortex flowmeter constitutes a commercially successful version of a vortex meter utilizing a two-section shedder to create a vortex street. It is an excellent flowmeter whose rate accuracy on low viscosity fluids, such as water, within a broad operating range is about 2%.
However, some flow rate measuring applications require a higher order of accuracy and still broader operating range. Also, in some applications the fluid being measured is subject to viscosity changes, turbulence and other disturbances which adversely affect the accuracy of the readings obtained with meters of the 10 LV 1000 type.
To provide a vortex-type flowmeter in which the frequency of vortex shedding is accurately related to fluid velocity regardless of turbulence, changes in fluid viscosity and other disturbing factors which tend to degrade this relationship, applicant's prior U.S. Pat. No. 4,030,355 (Herzl) discloses a flowmeter whose obstacle assembly is constituted by a fixed front section and a deflectable rear section cantilevered by beams from the front section, the rear section having a central opening therein to provide a fluid passage.
The front section of the Herzl patent flowmeter is contoured to cause flow separation of the incoming fluid, thereby dividing the stream to create a series of vortices that alternate with respect to the center line of the front section. As the vortices detach themselves from the front section, alternate areas of low pressure are created that shift from side-to-side, producing an oscillating thrust behind the front section and causing the deflectable rear section to swing periodically at a frequency proportional to the incoming fluid velocity. This swing is sensed by a strain gauge mounted on a beam from which the rear section is cantilevered.
The central opening in the rear section permits the flow of fluid therethrough and acts to smooth out turbulence behind the front section to a degree sufficient to create an orderly vortex trail straight down the center of the flow tube. This central passage significantly improves the accuracy and repeatability of the flowmeter.
In a vortex-shedding flowmeter of the Herzl patent type, the deflectable rear section is relatively heavy; and while this flowmeter has excellent hydraulic characteristics, it is quite sensitive to acceleration effects. Though it is possible to partially balance out these undesirable acceleration effects, some unbalance always remains.
Moreover, while the Herzl patent flowmeter design is generally effective in liquid flow rate measurement, it is not generally acceptable for metering gas flow. The reason for this limitation is that in liquid use, relatively large forces are generated by the vortices, whereas in gas flow measurement, the generated forces are smaller by many orders of magnitude, and the meter sensitivity is insufficient to respond effectively thereto, particularly if fading is encountered in the fluidic oscillations, as is sometimes the case.